I had originally used PNG's with my manifest, but changed them to JPG's for optimization. I changed the images in the image folder, but, did not go in the manifest to change the extensions to .jpg, lol, silly me! However, I did not catch the mistake, in my rush, because when testing my channel on my ROKU, all the images worked! I don't understand how they worked, but they did. If someone could explain that to me, I would be grateful... it is working badly on my curious mind!
One possibility is that you did not delete the PNG images from your .zip file, so your channel worked when side-loaded because it still had access to those images from the .zip file even though you had deleted them from your image folder.
Tip: Before submitting a package, delete your channel's .zip file then re-zip everything so your .zip file reflects the actual contents of your channel's directories, then complete your final pre-submission testing.